Sonar listening devices, commonly referred to as “sonobuoys”, are used to receive and transmit acoustic signals, process the data and communicate via radio frequency link. These devices are either passive (receive acoustic signals), or active (transmit acoustic signals). The basic components of an acoustic transducer array which is used for this purpose are “array elements”, each of which includes a transducer in the form of (for example) a piezoelectric ceramic that is electrically excitable to produce a “ping” as an acoustic waveform. The present invention applies to all sonar array configurations, including vertical and horizontal linear arrays, planar arrays and volumetric arrays, and to all transducer array element design technologies, such as, for example, flexural disc transducers.
An acoustic array for underwater applications generally includes a series of individual array elements at a predetermined spacing (typically on the order of 0.5 wavelengths) relative to each other. (See, for example, FIG. 15.) The acoustic performance of the array is determined by the acoustic response characteristics of the individual array elements and their relative spacing. Typically, the array resonant frequency is equal to the resonant frequency of the individual array elements, and the maximum radiated acoustic source level (that is, the sound pressure level) SPLarray of an acoustic array is equal to the summation of the maximum array element acoustic source level SPLarray element plus the directivity index (DI) associated with the spatial arrangement of the array elements in the array. That is, for an array of omni directional elements:SPLarray=SPLarray element+10*LOG10(number of array elements)+DIarray 
For maximum sonar performance it is desirable to maximize the amount of acoustic power generated from a given package volume. In addition, for sonar applications, lower frequencies are desirable because they have lower absorption losses and enhanced detection characteristics. The need for continued improvement of sonar performance at lower frequencies, together with a further requirement for an elevated maximum source level, typically mandates larger package sizes. In current high power, shallow water applications the maximum source level of an array element/array is limited by the onset of cavitation on the projector face. The cavitation threshold of a transducer is a well studied phenomena and is related to many factors. For a given transducer design/technology, the hydrostatic pressure on the face of transducer (operational depth) will determine the cavitation limited source level of the transducer. Increasing the depth of operation will increase the cavitation limited source level of the transducer.
The resonant frequency of a transducer is related to the stiffness and mass of the transducer as follows:Fr=(K/M)1/2 Where
Fr=resonant frequency for the transducer
K=stiffness of the transducer
M=mass of the transducer
In order to accommodate lower frequency requirements (that is, by reducing the resonant frequency of the array element/array), current practice is to decrease the stiffness of the transducer or to increase its mass. Reducing the stiffness is readily accomplished by increasing the size of the radiating element. Increasing the mass of the transducer can be done by various means but most common is the use of dense materials or the attachment of “added mass” to the radiating face of the transducer. However, all of these methods for reducing the resonant frequency of the transducer come at the expense of reduced transducer performance. The reduced stiffness will result in an increased package size and a transducer with decreased hydrostatic load capability. The increased mass reduces the Q factor of the system and results in reduced peak efficiency.
Accordingly, one object of the invention is to provide an acoustic transducer array and array element of the type described above, which achieves improved acoustic performance.
Another object of the invention is to provide such an acoustic transducer array and array element which can be operated with improved efficiency at a lower frequency, over a broader bandwidth, at higher source levels.
These and other objects and advantages are achieved by the acoustic transducer array and array element according to the invention, in which each transducer array element includes a plurality of individual transducer assemblies or “drivers” which are combined in a closely spaced geometry to form the array element. The respective transducer assemblies within each transducer array element are acoustically coupled with each other. The invention utilizes such mutual coupling between the closely spaced transducer assemblies to provide improved acoustic performance.
While the invention was verified with a flexural disc transducer technology, it is applicable to all transducer technologies. By spatially locating the transducer assemblies close to each other within each array element, the resultant array performance can be improved. That is, when two transducer assemblies that have individual resonant frequencies of Fr are spatially located in close proximity to each other (so that they are acoustically linked), the resultant resonant frequency of the new element will be on the order of 0.8*Fr. The invention thus achieves a desirable reduction in operational frequency, without reducing the stiffness or increasing the mass of the array elements, thereby avoiding the disadvantages associated with the prior art solutions referred to previously.
In addition, the invention also provides the following advances over the prior art single driver array elements:
Increased source level. With current transducer technologies, shallow water cavitation limited source levels are limited by the hydrostatic pressure (depth of operation) and the peak velocities that are reached on the surface of the projector face. This invention allows the array elements to be driven harder prior to the onset of cavitation. The internal mutual acoustic coupling preloads the face of the driver elements and allows the transducer to operate at increased source level when deployed in shallow water conditions.
Increased bandwidth. Current bandwidths of many transducer technologies are limited to the mechanical properties of the materials used to construct the transducer and the loading effects of the medium in which they operate. For instance, the Q factor of the transducer operating in air is much higher than the Q factor of the transducer operating in water. Additionally, the Q factor of a transducer built with materials with high internal damping characteristics is lower than a transducer built with standard materials. Known low Q factor, high bandwidth transducers typically possess low efficiencies since significant energy is dissipated in the damping materials. The present invention decreases the Q factor of the system without decreasing the overall efficiency of the transducer. The decreased Q factor results in a wider 3 dB bandwidth; it thus achieves easier electronic tuning of the transducer and facilitates operation with complex waveforms.
Decreased resonant frequency. Decreasing the resonant frequency of conventional transducers is currently accomplished by utilized larger projector surfaces, increasing the mass of the projector or decreasing the stiffness of the projector. These all have negative effects on the performance of the transducer. This invention decreases the effective resonant frequency of a given transducer element while maintaining the mechanical mass and stiffness of the individual driver design. This allows the transducer designer several options for a given design space. Resonant frequencies on the order of 20% allow the transducer designer to maintain mechanical integrity while achieving lower resonant frequencies. This reduces the mechanical stresses inherent in the operation of the transducer, which leads to high drive levels and deeper depth applications in non pressure compensated systems.
Increased depth applications. Current transducer depth limitations are directly related to the effects of hydrostatic pressure. In order to reach a target frequency in a given package size the transducer designer quite often sacrifices mechanical stiffness (and load carrying capability) for reduction in resonant frequency. This reduction in stiffness to meet the resonant frequency requirements reduces the ability of the transducer to withstand the effects of hydrostatic pressure, thereby decreasing its operational depth capability. This invention allows for reduced resonant frequency without sacrificing any mechanical integrity. For a given package space a stronger transducer with an equivalent resonant frequency can be realized. This stronger transducer has increased mechanical strength and is capable of withstanding higher hydrostatic pressure loads.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.